Generally, in the human heart, the sinus (or sinoatrial (SA) node typically located near the junction of the superior vena cava and the right atrium) constitutes the primary natural pacemaker by which rhythmic electrical excitation is developed. The cardiac impulse arising from the sinus node is transmitted to the two atrial chambers (or atria) at the right and left sides of the heart. In response to excitation from the SA node, the atria contract, pumping blood from those chambers into the respective ventricular chambers (or ventricles). The impulse is transmitted to the ventricles through the atrioventricular (AV) node, and via a conduction system comprising the bundle of His, or common bundle, the right and left bundle branches, and the Purkinje fibers. The transmitted impulse causes the ventricles to contract with the right ventricle pumping unoxygenated blood through the pulmonary artery to the lungs, and the left ventricle pumping oxygenated (arterial) blood through the aorta and the lesser arteries to the body. The right atrium receives the unoxygenated (venous) blood. The blood oxygenated by the lungs is carried via the pulmonary veins to the left atrium.
The above action is repeated in a rhythmic cardiac cycle in which the atrial and ventricular chambers alternately contract and pump, and then relax and fill. One-way valves, between the atrial and ventricular chambers on the right and left sides of the heart, and at the exits of the right and left ventricles, prevent backflow of the blood as it moves through the heart and the circulatory system.
The sinus node is spontaneously rhythmic, and the cardiac rhythm it generates is termed sinus rhythm. This capacity to produce spontaneous cardiac impulses is called rhythmicity. Some other cardiac tissue possess rhythmicity and hence constitute secondary natural pacemakers, but the sinus node is the primary natural pacemaker because it spontaneously generates electrical pulses at a faster rate. The secondary pacemakers tend to be inhibited by the more rapid rate at which impulses are generated by the sinus node.
Disruption of the natural pacemaking and propagation system as a result of aging or disease is commonly treated by artificial cardiac pacing, by which rhythmic electrical discharges are applied to the heart at a desired rate from an artificial pacemaker. A pacemaker is a medical device which delivers electrical pulses to an electrode that is implanted adjacent to or in the patient's heart to stimulate the heart so that it will contract and beat at a desired rate. If the body's natural pacemaker performs correctly, blood is oxygenated in the lungs and efficiently pumped by the heart to the body's oxygen-demanding tissues. However, when the body's natural pacemaker malfunctions, an implantable pacemaker often is required to properly stimulate the heart.
Implantable pacemakers are typically designed to operate using various different response methodologies, such as, for example, nonsynchronous or asynchronous (fixed rate), inhibited (stimulus generated in the absence of a specified cardiac activity), or triggered (stimulus delivered in response to a specific hemodynamic parameter). Generally, inhibited and triggered pacemakers may be grouped as “demand”-type pacemakers, in which a pacing pulse is only generated when demanded by the heart. To determine when pacing is required by the pacemaker, demand pacemakers may sense various conditions such as heart rate, physical exertion, temperature, and the like. Moreover, pacemaker implementations range from the simple fixed rate, single chamber device that provides pacing with no sensing function, to highly complex models that provide fully-automatic dual chamber pacing and sensing functions. For example, such multiple chamber pacemakers are described in U.S. Pat. No. 4,928,688 to Mower entitled “Method and Apparatus for Treating Hemodynamic Dysfunction,” issued May 29, 1990; U.S. Pat. No. 5,792,203 to Schroeppel entitled “Universal Programmable Cardiac Stimulation Device,” issued Aug. 11, 1998; U.S. Pat. No. 5,893,882 to Peterson et al. entitled “Method and Apparatus for Diagnosis and Treatment of Arrhythmias,” issued Apr. 13, 1999; and U.S. Pat. No. 6,081,748 to Struble et al. entitled “Multiple Channel, Sequential Cardiac Pacing Systems,” issued Jun. 27, 2000.
For example, a DDD pacer paces either chamber (atrium or ventricle) and senses in either chamber. Thus, a pacer in DDD mode, may pace the ventricle in response to electrical activity sensed in the atrium. Further, for example, a pacer operating in VVI mode, paces and senses in the ventricle, but its pacing is inhibited by spontaneous and electrical activity of the ventricle (i.e., intrinsic ventricular activity or events, wherein the ventricle paces itself naturally).
As such, it may be desired to sense in one cardiac chamber (e.g., detect electrical activity representative of contraction of a chamber and referred to as a “sensed event”) and, in response, pace (referred to as a “paced event”) in the same or different chamber. It also may be desired to pace at two electrode locations following a sensed event at one of the pacing electrodes or at a different electrode. For example, patients are often treated with pacemakers that include an electrode in each of the two atrial chambers and a third electrode in the right ventricle. Both atrial chambers usually are paced following a sensed event in either chamber.
Further, bi-ventricular pacing devices are also used for treatment of patients. For example, in such a bi-ventricular pacing apparatus, multiple implantable leads having electrodes associated with a part thereof are implanted to the respective chambers of a patient's heart and coupled to respective circuitry for forming multiple channels for pacing and sensing, e.g., left ventricular channel, right atrial channel, etc. Such an exemplary implantable, four-channel cardiac pacemaker is described in U.S. Pat. No. 6,070,101 to Struble et al. entitled “Multiple Channel, Sequential Cardiac Pacing Systems,” issued May 30, 2000. For example, the distal end of a right atrial lead is attached to the right atrial wall and a right ventricular lead is passed through a vein into the right atrial chamber of the heart and into the right ventricle where its distal electrodes are fixed. Another lead is passed through a vein into the right atrial chamber of the heart, into the coronary sinus (CS), and then inferiorly into the great vein to extend a distal pair of left ventricular pace/sense electrodes alongside the left ventricular chamber and leave a proximal pair of left atrial pace/sense electrodes adjacent the left atrium. With such electrode placement, pacing and sensing can be performed in each chamber of the heart, enabling bi-ventricular pacing. For example, such bi-ventricular pacing may be performed following atrial sensed events or atrial paced events.
Typically in such types of pacing apparatus, if an intrinsic or pacing pulse occurs in one of the chambers, for example, the atrium, then this activity may be erroneously sensed in the other chambers due to cross-talk. In order to eliminate this type of error, in the past, pacemakers have been provided with blanking periods for blanking the sensor in one channel after a pacing pulse occurs in the other. This blanking period is usually referred to as the cross-channel blanking period. Following the blanking period, an alert period is normally designated during which the cardiac chamber of interest is monitored for intrinsic activity. If no such activity is sensed by the end of this alert, then a pacing pulse is applied to the chamber. However, one problem with such pacemakers and the use of blanking channels has been selecting the duration of the blanking period for a particular channel properly. If the blanking period is too short, a cross-channel artifact could be interpreted as intrinsic activity and therefore pacing may be erroneously inhibited. On the other hand, if the blanking period is too long, intrinsic activity may be missed and the chamber may be paced when no such pacing is required. Either situation is undesirable physiologically.
Yet further, particularly in bi-ventricular pacing systems, e.g., systems which provide delivery of ventricular stimulus to both ventricular chambers following paced or sensed atrial events, a left ventricle lead is typically placed as described above, in a cardiac vein via the coronary sinus. Since the lead tip is in close proximity to the coronary sinus tractus, far-field coronary sinus/left atrial signals of significant amplitudes can potentially be sensed as ventricular activity and present inappropriate inhibition of bi-ventricular pacing. For example, in particular, when bipolar left ventricle leads are employed, the anode ring of the bipolar lead can be close to/or just within the coronary sinus system depending on tip-ring distance for the electrodes on the left ventricle lead. With the leads positioned in such a manner, atrial activity may be sensed using the left ventricle electrodes, taken as an intrinsic left ventricle event, and prevent or inhibit delivery of ventricular stimulus.
Further, for example, lead dislodgment may also lead to such mistaken sensing of ventricular events. For example, a left ventricular lead may be placed via the coronary sinus with a passive lead tip fixed in a cardiac vein. Leads are typically placed 1 to 4 centimeters within the vessels (or, generally, as far as possible). Either partial lead dislodgment (e.g., gradual pullback) or permanent lead dislodgment may result in an electrode location that is undesirable and conducive to over-sensing of left atrial activity. Therefore, once again, such over-sensing of atrial activity may lead to falsely sensed ventricular activity and the inhibition of the delivery of ventricular stimulus. As such, bi-ventricular stimulation may be intermittently or may be completely lost.
In many pacing apparatus, such as, for example, dual chamber pacing devices operating in DDD mode, ventricular safety pacing (VSP) is generally available and intended to prevent inappropriate inhibition of ventricular pacing by ensuring that an atrial paced event is followed by a ventricular paced event. When this VSP feature is on, ventricular sensing within a VSP window of typically 110 milliseconds following an atrial paced event causes ventricular pacing at the end of the VSP window (e.g., the 110 millisecond period).
For example, if the pacing apparatus is programmed with a paced AV interval (PAV=100 milliseconds) (i.e., the AV interval following an atrial paced event and defined as the time between the paced event and delivery of ventricular stimulus) that is less than the VSP window (e.g., 110 milliseconds), then the delivery of ventricular stimulus would occur at the end of the programmed PAV interval when ventricular sensing occurs during the VSP window.
In another example, if the pacing apparatus is programmed with a PAV interval (PAV=150 milliseconds) that is greater than the VSP window (VSP=110 milliseconds), then when ventricular sensing occurs during the VSP window, the delivery of ventricular stimulus would occur at the end of the VSP interval, and not at the time out of the PAV interval.
Further, with such conventional VSP, if no ventricular events are sensed during the VSP window and the PAV is greater in length than the VSP window, if a ventricular event is sensed during the PAV but after the VSP window, delivery of ventricular stimulus would be inhibited due to the sensed intrinsic ventricular event. This VSP feature is designed to ensure ventricular output in the event of noise sensing on the ventricular lead (e.g., cross-talk) within the VSP window or 110 milliseconds after an atrial paced event and outside the programmed ventricular blanking period.
Another example of a pacemaker having safety pacing is described in U.S. Pat. No. 5,782,881 to Lu et al., issued Jul. 21, 1998 and entitled “Pacemaker With Safety Pacing.” As described therein, a monitoring window is defined during an AV delay during which signals sensed in a ventricular channel are monitored. If an abnormal signal is sensed during this window, certain features of the signal are analyzed to determine if its origin is intrinsic or due to cross-channel activity or noise. Cross-channel activity is ignored. If intrinsic cardiac activity is identified, then no pacing pulse or ventricular stimulation is applied. If no decision can be made as to the source of the cardiac activity, then delivery of stimulus is performed and ventricular pacing is not inhibited by the sensed activity.
The above-described VSP features may be inadequate in many circumstances. For example, conventional VSP features typically only occur when programmed on, and only following a paced atrial event. In other words, a VSP window is only utilized following delivery of pacing stimulus in the atrium and thus during a PAV interval. Such VSP features do not occur following atrial sensing or an atrial sensed event, where a timed sensed AV interval (SAV) is initiated.
As indicated above, ventricular safety pacing is incorporated in many dual chamber devices to avoid inhibition of ventricular pacing due to ventricular oversensing of atrial signals after an atrial pace. Ventricular signals following an atrial pace within a timing window of about, for example, 70 to 110 milliseconds (i.e., VSP window) will trigger a ventricular pace which may, for example, be delivered at the end of the PAV interval or at the end of the VSP window to ensure capture of the ventricle.
Conventional ventricular safety pacing is beneficial in many circumstances, but not necessarily in all dual chamber devices. Particularly, such conventional VSP is not necessarily advantageous in atrially triggered bi-ventricular pacing where ventricular sensing is only performed by one ventricular electrode, e.g., a right ventricular electrode. For example, this is the case for the combination of an implantable cardioverter defibrillator (ICD) and an atrially triggered bi-ventricular pacer, such as the InSync ICD (Model 7272) available from Medtronic Inc.
In such a situation, a ventricular signal being sensed in the VSP window after an atrial pace (e.g., due to a premature ventricular contraction (PVC)), may initiate a safety pace, that, as it is delivered on both ventricular leads, can fall into the late refractory period on the left ventricular side. The possibility that the safety pace may be delivered in the late refractory period on the left ventricular side is due primarily to the asymmetry of sensing and pacing. For example, the chance of this occurring is greater when the sensing of the activity or event during the ventricular safety pacing window occurs far from where the actual event takes place.
For example, as shown in FIG. 7, a worst case scenario for the above situation is presented in the following illustrative case of sensing at the right ventricular side while providing bi-ventricular pacing. With sensing at the right ventricular side of the heart, there is the possibility that pacing of the left ventricular myocardium may occur as late as 220 milliseconds to 330 milliseconds after a premature ventricular contraction has occurred in the left ventricular myocardium. For example, a PVC 500 occurs on the left ventricular side of the heart. Thereafter, an atrial pace 501 is provided which initiates the ventricular safety pacing window 504. However, due to transmission delay, sensing of the PVC 500 on the right side of the heart does not occur until within the ventricular safety pacing window 504 as shown by sensed event 506. The sensing of the event 506 within the ventricular safety pacing window 504 commits to the provision of a ventricular safety pace 510 that as shown in the FIG. 7 can be as much as 330 milliseconds after the PVC 500.
Thus, there is an inherent possibility of inducing tachyarrhythmias using bi-ventricular safety pacing in such a situation. This may be particularly relevant when considering the increased rate of PVCs observed in the population having a primary incidence for an ICD together with the elevated intra-ventricular conduction delay in patients indicated for bi-ventricular pacing.
Table I below lists U.S. Patents relating to multiple chamber pacing devices and devices and methods having VSP features.
TABLE IU.S. Pat. No.InventorIssue Date4,890,617Markowitz et al.Jan. 2, 19904,928,688MowerMay 29, 19904,932,406BerkovitsJun. 12, 19904,944,298SholderJul. 31, 19905,144,949OlsonSep. 8, 19925,292,340Crosby et al.Mar. 8, 19945,318,594Limousine et al.Jun. 7, 19945,782,881Lu et al.Jul. 21, 19985,792,203SchroeppelAug. 11, 19985,893,882Peterson et al.Apr. 13, 19995,902,324Thompson et al.May 11, 19996,070,101Struble et al.May 30, 20006,081,748Strubie et al.Jun. 27, 2000
All references listed in Table I, and elsewhere herein, are incorporated by reference in their respective entireties. As those of ordinary skill in the art will appreciate readily upon reading the Summary of the Invention, Detailed Description of the Embodiments, and claims set forth below, at least some of the devices and methods disclosed in the references of Table I and elsewhere herein may be modified advantageously by using the teachings of the present invention. However, the listing of any such references in Table I, or elsewhere herein, is by no means an indication that such references are prior art to the present invention.